The exemplary embodiment relates generally to sheet transport systems and finds particular application in connection with a gravity-assisted registration system for sheet media which is suited to use in a printing system.
Transport systems are widely used for transporting sheet media between and within modules of a printing system, such as between a sheet feeder and a marking module, or on a return path through the marking module to enable duplex (double sided) printing. The transport system may include a combination of rollers, conveyor belts, vacuum-assisted transport units, and the like. In order to ensure that each sheet arrives at a printer component with an acceptable level of skew and lateral errors, registration subsystems are used to steer the sheets to achieve correct alignment.
There are demands for new printer designs that are able to increase the size of the sheet (e.g., to about 66 cm, or higher, in process-direction length) or to increase the printer speed from what is conventionally achievable. For the registration subsystem, steering long sheets and steering sheets at high speeds are challenging. In high speed printers, the amount of time available to perform the registration correction is reduced, which can increase stresses on the sheets. This means that sheets may not be correctly registered if their input error is too large. Registration correction algorithms are used to attempt to steer these sheets to the machine registration targets. However, the large corrections may result in sheet trailing edges being driven into the sidewalls of the transport (resulting in sheet damage or jams) or cause sheets to slip, breaking free of the drive nips, resulting in poor registration.
In one type of registration subsystem, skew and lateral errors are corrected in one motion. This correction induces more skew to move the sheet laterally, creating a ‘tail-wag’ motion of the sheet. This motion of the sheet is stressful, and the tangential forces on the sheet may exceed the threshold of slip with larger sheets. Another type of registration subsystem uses independent drive rolls for correcting skew while lateral correction is effected by a translating (cross-process) carriage. This has an advantage of decoupling the lateral and skew correction. However, the use of the translating carriage limits the maximum speed of the printer system due to the limit on the carriage return time that can be achieved, given the mass of the carriage (including motors, rollers and other drive elements).
One method used to enable registering large sheets is to manually adjust the positions of preceding modules to try to keep the input error low. For example, the sheet feeder may undergo an alignment procedure to reduce the errors in the sheet entering the marking module, or a duplex path alignment procedure may be performed. However, such module alignment procedures impact only the mean input error and are unable to address sheet-to-sheet variations. Thus, even though average input error may be within acceptable bounds, sheet-to-sheet variations can result in misregistration of some of the sheets.
There remains a need for systems and methods for media registration which address these deficiencies and enable improvements in the capability of a printing system to handle faster sheet speeds, larger sheet sizes, and/or larger weight sheets.